The following Patent Document 1 discloses a switching power supply device having a structure in which an IC detects the level of an input port IN and performs switching control on an FET connected to a primary coil of a transformer. Specifically, when the input port IN is at a high level, the IC stops the switching control of the FET. On the other hand, when the input port IN is at a low level, the IC starts the switching control of the FET to oscillate the transformer.